1. Field of the Invention
The present invention relates to a power unit for a vehicle. More specifically, the present invention relates to a power unit which converts the output of an internal combustion engine having an electromagnetic valve actuating device into electric power and the vehicle is then driven using the converted electric power.
2. Description of the Related Art
An electromagnetic valve actuating device for actuating intake valves and/or exhaust valves of an internal combustion engine is known in the art. An electromagnetic valve actuating device actuates valves of an engine by means of electromagnetic actuators without using camshafts.
Japanese Unexamined Patent Publication (Kokai) No. 8-93426, for example, discloses an example of this type of the electromagnetic valve actuating device. The device in the '426 publication includes an armature disk made of a magnetic material and is attached to a stem of a valve element and a pair of springs which urge the armature disk to the directions opposite to each other. The device further includes an opening solenoid disposed on one side of the armature disk and a closing solenoid disposed on the other side of the armature disk. When both of the opening solenoid and the closing solenoid are deactivated, the valve element is kept between a fully opened position and a fully closed position by the urging forces of the springs exerting on both sides of the armature disk. However, when the opening solenoid is activated, since the armature disk is pulled to the opening solenoid, the valve element moves to the fully opened position. Similarly, when the closing solenoid is activated, the valve element moves to the fully closed position.
Therefore, the valves of the cylinders of an internal combustion engine can be operated by the electromagnetic valve actuating device by activating the opening solenoid and the closing solenoid alternatively in the manner synchronous with the operating cycle of the cylinder. Thus, the valves of an internal combustion engine can be operated without using camshafts.
In general, a mechanical power loss in the valve system caused by friction etc. becomes large when a camshaft is used for driving valves. Since the mechanical power loss of an electromagnetic valve actuating device is very small compared to that of a camshaft, the fuel efficiency and the performance of the engine can be improved by using an electromagnetic valve actuating device.
However, although the power loss of the electromagnetic valve actuating device is small, some problems arise when it is used for an engine of a vehicle.
For example, the vehicle engine is required to operate over a wide speed range from a very low speed to a very high speed. Since the electromagnetic valve actuating device drives a valve element having a mass by an electromagnetic actuator such as a solenoid, the actuating speed of the valve is limited. Therefore, when the engine is operated at a high speed, the electromagnetic valve actuating device cannot achieve a sufficient actuating speed and the movement of the valve elements cannot match the speed of the cycle of the cylinders.
Further, the performance of the electromagnetic valve actuating device, such as the durability and the reliability in the continuous high speed operation and response in the transient speed condition, is not sufficient at present.